Generally, the small computer system interface (SCSI) is used for more easily installing a peripheral device to a computer system. In the SCSI device, the interface card is called a SCSI host adapter or a SCSI controller and the peripheral device is called a target.
FIG. 1 is a block diagram illustrating a conventional SCSI system.
As shown therein, the conventional SCSI system includes initiators 10 for generating an input/output command, targets 20 for performing a commanded input/output operation in accordance with the control of the initiators 10, respectively, and a SCSI bus for forming a transmission path between the initiator 10 and the target 20.
The initiator 10s include host computers 12a and 12b, and SCSI controllers 14a and 14b. The target 20 includes a plurality of peripherals such as a printer 22a, a CD-ROM drive 22b, a scanner 22c, etc.
When transmitting data over the SCSI bus, there are eight phases involved. There are a bus free phase, an arbitration phase, a selection phase, a reselection phase, a command chase, a data phase, a status phase, and a message phase. Two phases may not exist at the same time. The communication, e.g., a data transmission, is performed by a combination of the above-described phases.
Here, the command phase, the status phase, and the message phase are called information transmission phases, and the SCSI controllers 14a and 14b use the SCSI protocol of the information transmission phase to drive the SCSI bus.
Under the SCSI protocol, the SCSI control signal is communicated from one host computer, e.g., 12a, to the other host computer 12b and to one of the peripherals 22a, 22b, 22c, and 22d of the target 20 through the SCSI bus. The direction of the signal transmission is controlled by the target 20.
The SCSI control signals (SCSI.sub.13 CTL) include eighteen signals designated as BSY, SEL, C/D, I/O, MSG, REQ, ACK, ATN, RST, DB&lt;7: 0, P:&gt;7. These signals are discussed below in more detail.
SEL: A signal which is used when the initiator selects the target, or the target selects the initiator.
C/D: A signal which indicates whether the data on the data bus is related to a command or to data provided by the target.
I/O: A signal which indicates the direction of the data and is provided by the target.
MSG: A signal which is given by the target during the message phase.
REQ: A complementary signal of a REQ/ACK handshake, which signal is provided by the target.
ACK: A signal of an REQ/ACK handshake, which is provided by the initiator.
ATN: A signal which is provided by the initiator for indicating an attention state of the target.
RST: A resetting signal.
DB&lt;7: 0, P&gt;: 7 data bus bits and one parity bit.
There are problems with the conventional SCSI system. In order for the host computer 12a or 12b to test the SCSI controller 14a or 14b, respectively, a target 20 must be connected to the SCSI controller 14a or l4b. Therefore, during a testing operation, the set-up time and cost are increased.
To test a target for data corruption, the host controller must pass data through the SCSI controller. This technique assumes that the SCSI controller is not the source of the data corruption, but this is not always a valid assumption. Hence, there is a problem that a malfunctioning SCSI controller could cause a host computer to misdiagnose a target as the source of data corruption.